Conventional III-Nitride based HFET structures are typically fabricated on a gallium nitride (GaN) buffer layer. As device scaling geometries are reduced to achieve higher frequency applications, short channel effects need to be minimized. To improve the carrier confinement and sub-threshold characteristics, a back-barrier may be added before the channel layer. Typically, aluminum gallium nitride (AlxGa1-xN) alloys have been used as the back-barrier due to their wider band-gaps than the GaN channel. In most cases, these alloys comprise the buffer layer and are not just an added layer to the structure. However, there are several limitations related to using AlGaN alloys. Specifically, in conventional alloys, the aluminum (Al) composition normally needs to be kept low, on the order of less than 8%, to avoid degradation of crystalline quality under normal growth conditions. Additionally, the thermal conductivity of the buffer may be degraded as the aluminum concentration is increased.
In some cases, GaN based HFETs typically have AlGaN as the barrier layer. To scale down the gate length for high frequency applications and minimize short channel effects, the thickness of the barrier layer needs to be scaled down accordingly. For example, a gate length less than 0.30 microns (μm) is desired for high frequency application with a barrier layer thickness less than 200 Angstroms (A). However, with traditional AlGaN barriers, such scaling becomes unsuitable. To obtain high channel conductivity with a thin barrier, the Al concentration in the barrier needs to be increased significantly. This increase in Al concentration may result in enhanced alloy scattering that degrades the electron mobility. Furthermore, the crystal quality of AlGaN with high Al concentration may be poor, which may not be desirable for fabricating highly reliable devices. Though other approaches like indium aluminum nitride (InAlN) have been tried, the breakdown voltage and reliability are limited due to poor crystalline quality.